This invention relates to the field of sewage treatment.
In the treatment of sewage, it is standard practice to initially strain the sewage to remove coarse materials and large objects, followed by transfer of the sewage to a settling basin, where sand is allowed to settle out.
The sewage is then transferred to a primary basin forming a thickened or "heavy" settled sludge. The settled sludge is pumped to air flotation basins where it separates from and floats on top of a heavier layer which contains mostly water with a very small (less than 1%) amount of solids. The thickened sludge layer contains greater than 2% by weight, and generally from 5.5 to 7% by weight solids, in addition to water and air. Because this sludge contains a very large amount of air, its density is generally only in the range of about 0.06 to 0.1 grams per ml.
The thickened sludge layer which generally forms to a depth of 24 to 36 inches, is skimmed off by raking, and is subsequently digested and incinerated. The liquid layer in the primary basin is removed and subjected to a biological treatment to form activated sludge which is mixed with the raw sewage entering the treatment cycle.
In the treatment process, it is very important to know the thickness of the sludge in the air flotation basin, in order to operate the process at its maximum efficiency. If the thickness of the sludge becomes too great, it is necessary either to increase the time of operation of the rakes removing the sludge, or to slow down the entry of the sludge into the primary basin. The sludge, however, is a very difficult material to work with. The sludge is of variable consistency, smells bad and presents a health hazard since its bacterial levels are so high. It cannot be determined by looking from above what the actual thickness of the sludge layer is.
U.S. Pat. No. 3,923,655 discloses a system for detecting the level of sludge in a sedimentation tank in which a heavy sludge layer settles to the bottom, below a lighter liquid layer. According to this method, a series of temperature indicating devices are disposed in the tank, these devices indicating a temperature differential between the sludge and the liquid.
U.S. Pat. No. 4,436,630 discloses an apparatus and method for separating a mixture of two liquids of different densities utilizing a series of floats disposed at different levels in a tank. The two lower floats are filled with the anticipated less dense liquid or otherwise preweighted to a predetermined specific gravity.